Amplifier for wave-form signals



' July 22, 1941. w, w Y 2,250,144

AMPLIFIER FO R WAVE-FORM SIGNALS Filed April 8, 1940 Patented July 22,1941 All'lPLIFlER FOR WAVE-FORM SIGNALS William R. Welty, San Antonio,Tex., assignor to Olive S. Petty, San Antonio, Tex.

Application April 8, 1940, Serial No. 328,571

12 Claims.

This invention relates to improvements in apparatus for the receptionand amplification of wave-form electrical signals, and is especiallyconcerned with the elimination of disturbances which produce noise andthereby interfere with the reception of the desired signal, for instancethe disturbances resulting from static and other electrical discharges.

It has been heretofore proposed to provide control circuits for radiobroadcast receivers to effect blocking of the receiver on the arrival ofnoise impulses, for example as suggested in the patents to Lamb No.2,101,549 and Burrill No. 2,151,739. However, the control circuitsdisclosed in these patents are constructed to function in response to asignal which exceeds a predetermined amplitude, and thus may, in thecourse of reception of signals from different transmitting stationshaving different field strength, and from stations of comparable fieldstrength which transmit carrier frequencies employing differentmodulation percentages, actto suppress desired signals in the absence ofnoise disturbances, provided the received signals exceed thepredetermined limit.

It is well recognized that energy resulting from noise disturbances ispropagated by waves having sharply sloping Wave-fronts. In other words,the rate of change of energy on the arrival of a noise disturbance isquite high, as compared with the rate of change of normally modulatedcarrier wave signals. Thus it is possible to discriminate between thewanted and the unwanted signals by providing a device which isresponsive to rate of change of signal energy, and to apply the outputof such a device to the momentary suppression of the receiver.

It is therefore an object of the instant invention to provide, inconjunction with a receiver or amplifier for wave-form signals, a devicefor differentiating incoming signal energy with respect to time, and toapply the derivative thus obtained to the suppression of the receiver.

More specifically, it is an object of the invention to associate with areceiver for wave-form signals, a differentiating device, and a deviceenergized by the output of said differentiating device, and responsiveonly when such output exceeds a predetermined minimum, for applying acontrol bias to the receiver to minimize the disturbance by momentaryinterruption of the normal functioning of the receiver. Preferably theinterval of interruption of transmission or reproduction of signalenergy in the receiver is determined by a time-delay device, theinterval being sulficiently short to pass unobserved in the case ofaudible reproduction of music, speech or the like signals.

Control is preferably applied to the receiver, in the case of receptionof modulated carrier wave signals, at the audio frequency amplifier.Thus, in apparatus heretofore proposed for the control and suppressionof noise energy, the suppressing signal has been applied to the carrierwave before demodulation, with the result that an audible disturbance,quite annoying to the listener, has been produced. Again, with a view tofurther reduction of audible disturbance, the control signal ispreferably applied in the form of a more positive grid bias to athermionic amplifier valve which is normally operated near the upperportion of the grid voltage-plate current (Eg-Ip) curve, whereby thevariation in current output is materially reduced, and further reductionof noise is made possible.

Further objects and features of the invention will be apparent from thefollowing description taken in connection with the accompanying drawing,in which Figure 1 is a diagrammatic representation of one form ofreceiver circuit to which the instant invention may be applied; and

Figure 2 is a curve representing the operating characteristics of avalve to which control is applied.

In order to facilitate an understanding of the invention, reference willbe made to the several embodiments thereof illustrated in theaccompanying drawing and specific language will be employed. It willnevertheless be understood that various further modifications of thedevices illustrated herein, such as would fall within the province ofthose skilled in the art to construct are contemplated as part of thepresent invention.

The invention has been illustrated as applied to a simple form of tunedradio frequency receiver, but the application of the invention to otherforms of the receiver, for example the superheterodyne receiver, will beobvious.

Thus the incoming signal is fed through primary winding It of atransformer H, the secondary winding I2 of which is broadly tuned to thedesired carrier frequency by variable condenser !3. The signal voltageis then applied to the grid l5 of a thermionic valve l6, grid biasvoltage being supplied by a source IS. The anode 2b of valve 16 isconnected to one end of the primary winding 2| of a transformer 22,tuned by a variable condenser 24. The secondary winding 25 oftransformer 22 is tuned by a variable condenser 26, and applies controlvoltage to the grid 29 of a thermionic valve 30, grid bias voltage beingsupplied by a source 3|. The output of valve 30 is fed to a transformer34, the primary winding of which is tuned by a variable condenser 35,the output of transformer 34 being delivered to a thermionic valve 33,which functions as a detector, a grid leak resistance 39 and condenser40 being employed.

It will be appreciated that the circuit thus far described is entirelyconventional; other types of amplification may be employed in lieu ofthe cascade tuned circuits shown, and other methods of detection ordemodulation may be used. The details thus far described constitute inthemselves no essential part of the instant invention.

The output of detector valve 38 is applied to an audio frequencytransformer 42. Thus anode 43 of valve 38 is connected to one end of theprimary winding 44 of the transformer, a bypass condenser 45 beingshunted across the anode 43 and the cathode 46 of the valve. Secondarywinding 48 of transformer 42 is connected in a push-pull circuitincluding thermionic valves 49 and 59, the opposite ends of winding 48being connected to the respective grids 52 and 53 of the valves. Theanodes 54 and 55 of the push-pull valves are connected to the oppositeends of the primary winding 58 of transformer 50, the secondary winding59 delivering voltage to the grid 83 of valve 64;.the output of thisvalve is fed to the output transformer 68, with which is associated areproducer or speaker 10.

It will again be appreciated that the arrangement just described for theamplification of signal energy following detection is conventional, andmay be replaced by other known amplifying units. One method of applyingthe invention to a receiver or amplifier of the type illustrated willnow be described. 1

Part of the energy output of the detector valve 38 is delivered to'adifferentiating or derivative taking circuit indicated generally at 15,this circuit being associated with a thermionic valve 15. Thus plate 43of valve 38 is coupled through a condenser l! to the grid 18 of valve'56, the grid circuit including a resistance 79 and a source of gridbias potential including a potentiometer 8! in shunt with a bias battery82. v

The value of resistance 79 and the capacity of condenser 11 are quitesmall, so that the voltage drop across the condenser 11 is far greaterthan across resistance 19. Accordingly, the voltage applied to grid '58is substantially proportional to or representative of the derivativewith respect to time of the total voltage applied across the derivativetaking circuit 15. Other methods of applying to the grid 78 of valve 16a voltage which is generally representative of the rate of change ofincoming signal energy may replace the method chosen for illustration,and are contemplated by the present invention.

The potentiometer 8| is so adjusted that when desired signal energy onlyis being received, the negative bias on grid 18 of valve 16 issufficient to block the valve. When, however, signal energycharacterized by a sharply sloping wave-front, such as that resultingfrom static or other electrical disturbance, is impressed on thereceiver, the differentiation of the energy effected by circuit 15results in the application to grid 18 of a much less negative bias, andcurrent flows in the anode circuit of valve 16. It will be observed thata resistance 84 and a condenser in shunt therewith are interposedbetween the cathode 81 of valve 16 and the ground, and thus the voltagedrop across resistance 84 resulting from the flow of anode current isapplied to condenser 85, discharging through the resistance. Cathode 81is connected to auxiliary control grids 89 and 99 of valves 49 and 50respectively in the push-pull audio circuit hereinbefore described. Thuswhen valve 18 responds to the arrival of noise energy, a positive bias,representing the voltage applied to condenser 85, is applied toauxiliary control grids 89 and 90. The interval during which this biasis applied is determined by the characteristics of the time-delay deviceincluding the resistance 84 and condenser 85, and is preferably aboutone-twentieth of a second, and during this interval valves 49 and 59 areblocked, so as to suppress signal energy flowing in the receiver. Normalpersistence of hearing is such that suppression of signal energy forsuch a short period of time is not appreciated by the ear. If the timeconstant of the time-delay device including the resistance 84 andcondenser 85 is too long, the silence is noticeable, and while not asdisconcerting as static, may nevertheless be a source of slightirritation. If, on the other hand, the time constant is too short, itwill fall within the audible range, and unless all constants are atcritical values will produce a decided thump in the speaker.

The preferred method in which the voltage applied to the auxiliarycontrol grids 89 and 90 is employed to silence the receiver isillustrated graphically in Figure 2 of the drawing, which represents theEg-I curve of the valves 49 and 50. The normal operating point isindicated at 0, near the upper portion of the curve. When voltage issupplied to the auxiliary grid of the valve, the operating point isdisplayed approximately to S, which lies on the substantially horizontalportion of the curve, so that the output of the valve is minimized orreduced to zero. Control of the valves in this manner requiresconsiderably less voltage to effect the necessary reduction of outputand reduces substantially the variation in plate current of the valveswhich must be balanced out in transformer 69. Thus if valves 49 and 50are not identical in characteristics, suppression of the output bydisplacing the operating point to the lower portion of the E Ip CHIVBinevitably produces a pop in the speaker. Since little change in platecurrent is required, however, to displace the operating point from O toS, noise due to suppression is materially reduced or eliminated.

It is, of course, possible to apply the controlling or suppressingsignal otherwise than as specifically disclosed herein, and in thebroader aspect of the invention, such further modifications arecontemplated.

It'is advisable to employ radio frequency circuits in which the directcurrent resistance from the control grids is relatively low, in order toprevent blocking of the valves due to large incoming disturbances. It isalso desirable, in the event automatic volume control is employed, toapply the control voltage to a separate control grid in order that along time constant may be employed in the volume control circuit withoutdanger of blocking the valve.

It will be appreciated that the foregoing circuit is capable of use invarious fields Where it is helpful to eliminate undesired energy havinga sharply sloping wave front. For example, in

the field of seismic surveying, it can be shown that energy which hastraveled through very little soil is characterized by a much steeperwave front than energy which has traveled appreciable distances throughthe ground. The latter energy, and particularly that which is reflectedfrom strata at considerable depths, is most significant in determiningcontours, and a circuit of the type shown herein for suppressing signalenergy on the arrival of sharply sloping waves can readily be applied tothermionic valve amplifiers of the type commonly used in seismic Work,with the result that the less desirable Waves are substantiallyeliminated from the record.

Having thus described the invention, what is claimed as new and desiredto be secured by Letters Patent is:

1. In a receiver for wave-form signals, the combination with signalamplifying means, of means for suppressing the output of said amplifyingmeans, and a signal derivative-taking device responsive to noise energyfor initiating operation of said last named means.

2. In a receiver for wave-form signals, the combination with signalamplifying means, of means for suppressing the output of said amplifyingmeans, and means for converting incoming energy into a formrepresentative of the rate of change of such energy for energizing saidsuppressing means.

3. In a receiver for wave-form signals, the combination with signalamplifying means, of means for suppressing the output of said amplifyingmeans, a signal derivative-taking device responsive to noise energy forinitiating operation of said last named means, and time-delay means fordetermiing the period of operation of said suppressing means.

4. In a receiver for wave-form signals, the combination with signalamplifying means, of means for suppressing the output of said amplifyingmeans, means for converting incoming energy into a form representativeof the rate of change of such energy for energizing said suppressingmeans, and time-delay means for determining the period of operation ofsaid suppressing means.

5. In a receiver for wave-form signals, the combination with anamplifier for the signals, of means for suppressing signal energy insaid amplifier, said last named means comprising a separate path forsignal energy, including a signal derivative-taking device.

6. In a receiver for modulated wave-form signals, the combination withmeans for interrupt" ing the output of said receiver, of means operableon arrival at said receiver of noise ener y and responsive to thesharply sloping wave-front of such energy for initiating operation ofsaid interrupting means, said last named means including a signalderivative-taking device.

7. In a receiver for modulated wave-form signals, the combination withmeans for interrupting the output of said receiver, of means operable onarrival at said receiver of noise energy and responsive to the sharplysloping wave-front of such energy for initiating operation of saidinterrupting means, said last named means including thermionic valvemeans affording no appreciable response to signal energy below apredetermined amplitude, and a signal derivativetaking device deliveringenergy to said valve means.

8. In a receiver for modulated wave-form signals, the combination withmeans for interrupting the output of said receiver, of means operable onarrival at said receiver of noise energy and responsive to the sharplyslop-ing wave-front of such energy for initiating operation of saidinterrupting means, said last named means including thermionic valvemeans aifording no appreciable response to signal energy below apredetermined amplitude, a signal derivative-taking device deliveringenergy to said valve means, and time-delay means for determining theinterval of interruption of receiver output.

9. In a receiver for modulated wave-form signals, including radiofrequency and audio frequency amplifying circuits, the combination withmeans associated with the audio frequency circuit for interrupting theoutput of said receiver, of means operable on arrival at said receiverof noise energy and responsive to the sharply sloping wave-front of suchenergy for initiating operation of said interrupting means, said lastnamed means including a device for differentiating incoming signalenergy with respect to time.

10. In a receiver for wave-form signals, the combination with athermionic valve amplifier, of means for momentarily suppressing theoutput of said amplifier in response to the arrival of noise energy,said means including a device for electrically differentiating incomingsignal energy with respect to time to afford an output representative ofrate of change of the energy, of means operable by said diiferentiatedenergy and appreciably responsive only to energy above a predeterminedlevel for applying to said amplifier a suppressing grid bias.

11. In a receiver for wave-form signals, the combination with athermionic valve amplifier, of means for momentarily suppressing theoutput of said amplifier in response to the arrival of noise energy,said means including a device for electrically differentiating incomingsignal energy with respect to time to afford an output representative ofrate of change of the energy, of means operable by said differentiatedenergy and appreciably responsive only to energy above a predeterminedlevel for applying to said amplifier a suppressing grid bias, and meansassociated with said last named means for determining the suppressioninterval.

12. A method of treating modulated wave-form signal energy to eliminatestatic disturbances and the like, which includes the steps of amplifyingand reproducing the wave-form energy, taking a derivative of a portionof the energy with respect to time to obtain a signal of increasedamplitude on the arrival of noise energy, and applying the signal ofincreased amplitude to momentarily interrupt the amplification andreproduction of the signal energy.

R. WELTY.

